Conventionally used bearing steels are described below.
Rolling bearings such as ball bearings and roller bearings, which have a contact surface strength in a range of 1000 to 1300 MPa or in a range of 3000 to 4000 Mpa, are formed of high-carbon chromium bearing steel having a high content of carbon or case-hardened steel having a carburized surface. The high-carbon chromium bearing steel contains main components of 1.1% by weight of carbon and 1 to 1.5% by weight of chromium and varies its quenching characteristic according to the content of manganese and molybdenum. This type of steel is quenched at a temperature of 1050 to 1120K, and then tempered at a temperature of 420 to 470K to produce a texture with 7 to 8% by weight of spherical cementites dispersed in martensite. Since the hardness of the tempered steel is as high as HRC 58 to 64 by Japanese Industrial Standard (hereinafter referred to as JIS), clean steel having less flaws and less non-metal inclusions is desirable. At present, the high-carbon chromium bearing steel is typically manufactured utilizing deoxidization by carbon under vacuum degassing process. Further, special melting processes such as electroslag remelting or vacuum arc remelting are combined to reduce non-metal inclusions and to provide fine texture.
The carburized bearing is manufactured by carburizing case-hardened steel, and therefore, has high surface hardness and flexible core portion. The carburized bearing is especially suitable for use as a bearing which is subjected to an impact load.
In case of bearings used under a temperature which is above 390K, the texture of steel tempered at a low temperature is subject to variation, thereby resulting in softening or variation in dimension, and thus, the steel becomes unusable. For this reason, high-carbon high-alloy steels which are tempered at high temperatures, such as M50 (0.8 wt % C—4 wt % Cr—4.3 wt % Mo—1 wt % V) or T1 (0.7 wt % C—4 wt % Cr—18 wt % W—1 wt % V) are used.
However, the conventional bearing steels suffer from drawbacks as described below.
The case-hardened steel is difficult to reduce oxygen content in terms of melting and refining in contrast to the high-carbon chromium bearing steel and tends to generate oxide-based non-metal inclusions, which may reduce rolling contact life.
In addition, the high-carbon high-alloy steel tends to generate large-sized carbides, which may also reduce rolling contact life of the bearing.
On the other hand, the high-carbon chromium bearing steel does not have such drawbacks and can obtain high processing precision. Therefore, the high-carbon chromium bearing steel is suitable for use as the rotating portion of the precision instrument which particularly requires noiselessness during rotation. However, the high-carbon chromium bearing steel tends to rust and needs to be coated with rust-proof oil on an outer surface thereof. The rust-proof oil may be gasified and cause a malfunction of the precision instrument.
Accordingly, martensite based stainless steel corresponding to SUS440C steel by JIS with high corrosion resistance and high wear resistance is used for the bearings used in corrosive atmosphere. However, this stainless steel contains eutectic carbides resulting from an eutectic reaction when molten steel is solidified or non-metal inclusions such as alumina resulting from a chemical reaction of impurities of a material in the molten steel. When this stainless steel product is cut, high-precision cutting action cannot be achieved due to a difference in cutting state (machinability) of texture between the eutectic carbides or the non-metal inclusions and the stainless steel product. In particular, since rolling contact grooves formed on inner and outer rings cannot be processed with high precision, the rolling bearing vibrates and generates a high level of noise during rotation. Therefore, this stainless steel product cannot be used for the rotating portion of the precision instrument.
Accordingly, there has been proposed a rolling bearing which can improve noiselessness, and provides high wear resistance and high corrosion resistance (for example, see Japanese Unexamined Laid-Open Patent Publication No. Hei 6-117439 and Japanese Patent Publication No. Hei 5-2734.
The Publication No. Hei 6-117439 discloses a ball bearing comprising a plurality of balls made of high-carbon chromium bearing steel interposed between inner and outer rings, at least one of the inner and outer rings being formed of martensite based stainless steel having hardness of HRC 58 or higher by JIS and comprising eutectic carbides having a diameter of 10 μm or less.
The Publication No. Hei 5-2734 discloses a rolling bearing formed of stainless steel comprising a plurality of rolling elements interposed between inner and outer rings, the stainless steel comprising carbon of 0.6 to 0.75 wt %, silicon of 0.1 to 0.8 wt %, manganese of 0.3 to 0.8 wt %, chromium of 10.5 to 13.5 wt %, iron as remaining component, and impurities inevitably incorporated thereinto, and containing eutectic carbide with a long diameter of 20 μm or less and an area ratio of 10% or less.
When large-sized eutectic carbides appear on the surface of the bearing, a proper finished surface is difficult to form, due to the difference in cutting state between these carbides and a matrix around the carbides, and a noise may be generated during rotation, as described above. In addition, since the large-sized eutectic carbides generate a difference in wear resistance between the carbides and the matrix around the carbides during use of the bearing, they drop from a cracked surface, which causes a deformation of the surface shape of the bearing and significantly degrades noiselessness. It is therefore desirable to minimize the size of the carbides, because they are less likely to appear on the surface of the bearing. So, reducing the diameter of the carbide to 20 μm or less or 10 μm or less as disclosed in the above publications is effective in lowering the level of noise. Nonetheless, a satisfactory noiselessness cannot be obtained merely by reducing the size of the carbides. In addition, additional processes in manufacturing technique are required to thus reduce the size of the carbides. This greatly increases a manufacturing cost and is therefore unpractical.
The present invention has been developed in view of the above described problems accompanied by the prior arts, and an object thereof is to provide a rolling bearing which can improve noiselessness, and provide high corrosion resistance and longer life (corresponding to high wear resistance) and can be manufactured at a low cost, a material for the rolling bearing, and an instrument including a rotating portion using the rolling bearing.